I have some friends, and they say “Yo!” when I call them. I haven”t heard this response until quite recently (somehow), and I thought it was some word coined by rappers in their songs, and was adopted as a response by other people (like my friends, who happen to listen to rap).

So I was rather surprised, when looking this up on Wiktionary, I read:

(military slang) Present! Here!

Sergeant: Smith? Private Smith: Yo!

I thought to myself, “Obviously this ain’t coined by them rappers!”

Then, I saw a similar question on Yahoo Answers, with an answer stating (one link provided):

Yo is an American English slang interjection. The origins of the word may possibly be traced back to 14th century England. However, it was highly popularized after being commonly used among Italian Americans and African Americans in Philadelphia, Pennsylvania.

Clicking on the link provided, it turns out to lead to Answers.com. But the funny thing was, the new source included this information:

An exclamation used as a greeting, to express excitement, to attract attention, or as a general sign of familiarity (originating among young African-Americans). (1966–) .

I am inclined to think that the word yo came from the 14th century, but have no real evidence to say so. Can anyone provide solid stuff? That is, did it come from a military response, or a word used among young African-Americans in 1966?

Edit: I am asking specifically in relation to yo being used as a response/reply.

10 Answers
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I think that any etymology of "Yo!" that goes back only a few hundred years is woefully incomplete and quite absurd.

"Yo!" is used in more-or-less formal situations in East Asia (China, Japan), India (Dravidian languages), Africa (West and Central Africa), the United States, and Europe. That usage range puts it well beyond the purview of Indo-European, and suggests that its origins could lie entirely outside any formal etymology - but if it does have an origin, it obviously ain't English (as your source up there says, suggesting it may have come from Africa, or the Mediterranean, or both).

Arguing that this simple sound is derived from "an exclamation" back in AD 1400 is saying nothing more than "Back then, in AD 1400, nobody knew where it came from, either." Compare, for instance, the exclamation "Zounds!", which has a certain date of origin, and a certain meaning from which it is derived: "Yo!" has none of that.

Basically, "Yo!" is a simple sound that gets used a lot, around the world; so long as it's not a formal word in one's local language, it will tend to get used for more-or-less formalized exclamatory purposes. This makes sense because it's A) easy to say, B) the sounds occur in pretty much any language on Earth, and C) the sounds carry a quite a way's distance, and are easily distinguished from other sounds and words.

In the US, it was re-purposed as a greeting and response by Af-American culture some time in the late 60's, or so, and that's the answer you really want, here. It may have been absorbed into Af-American culture through Basic Training in the US military, during Vietnam (or WWII, as suggested by the other poster, above), or it may be a holdover from something more ancient, perhaps an African dialect; it's to answer questions like this that the idea of "ebonics" was once promoted. I have no idea if that discipline -- if it can be called that -- is still around or not, but that might be a good place to start if it is. In any event, it appears that currently linguists just can't really give your question any definitive answer.

There are actually a few different meanings reported in the OED for "yo" as an interjection, or a response:

An exclamation of incitement, warning, etc. (also repeated). In nautical use = yoho int. Occas. as n. and in vbl. n. yo-yoing.

first noted use: 1420

An exclamation used as a greeting, to express surprise, or to attract attention; hey! Also as a response to such a call. slang (orig. and chiefly U.S.).

first noted use: 1958

slang (orig. in African-American usage). In weakened use, following or punctuating an utterance for emphasis or as a general conversational filler.

first noted use: 1987

So the OED says that yo can be traced back to the 1400s, though not the 14th century. As an interjection, it has been used as a warning or emphasis, and weakened within the last 30 years. It has fallen into different groups (nautical, US slang, African American slang) but its current use remains tied to its 1400s use. As many words change in meaning over time, it seems well-founded to say that yo is from circa 1420.

It is possible that yo is even from the 14th century, and was only recorded in 1420. As a solid date for which the interjection was in use, however, 1420 is a sourced answer.

as a greeting, 1859, but the word is attested as a sailor's or huntsman's utterance since early 15c. Modern popularity dates from World War II (when, it is said, it was a common response at roll calls) and seems to have been most intense in Philadelphia.

I lived for four years in Philly, and can vouch that this is indeed what they use there where most of the rest of the USA uses "Hey", and perhaps Brits use "Oi". A native told me a funny story about trying to get someone's attention in upsate New York for a good minute or two before he realized the problem and switched to "Hey!"
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T.E.D.Sep 2 '11 at 13:47

It dates back to early man even before formalized spoken word. In today's zoological studies the sound has been picked up when animals first come in contact with one another so the sound that yo makes is primal.

I am rereading and teaching Keroac's On the Road and one of the minor characters uses it as a greeting. That part of the book is set in 1947.

"Good-by, Ray. When do we meet again?" I went to look for Carlo and Dean -nowhere to be found. Tim Gray shot his hand up in the air and said, "So you're leaving, Yo." We called each other Yo. "Yep," I said. The next few days I wandered around Denver.

The character is Tim Gray, who is based on Edward White, a friend of Keroac's from Columbia, who was born and raised in Denver. He served in the navy during the war, which makes sense.

Nice find! I've edited in a quote, feel free to change it or add another. The book was completed in 1951 and first published in 1957. I wonder if it's also in any drafts. On the Road: The Original Scroll was published in 2007 and is a "slightly edited version of the original manuscript".
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HugoFeb 7 '13 at 18:14

One of the changes in The Original Scroll is the original names are used. And it appears the passage is there: Ed White shot his hand up in the air and said “So you're leaving Yo.” We called each other Yo. “Yep,” I said.
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HugoFeb 7 '13 at 18:39

A good summary on some contemporary research on the subject is at http://sbp.so/yo.

That article also mentions a New York Times letter-to-the-editor by Ernest Paolino in which he descibes the possible Neopolitan-Philadelphian origins of the term. This seems quite plausible.

From the letter:

In the Neapolitan dialect "guaglione" (pronounced guahl-YO-nay) signified a young man. The chiefly unlettered immigrants shortened that to guahl-YO, which they pronounced whal-YO. That was inevitably further shortened to yo. The common greeting among young Italian-American males was "Hey, whal-YO!", and then simply, "Yo!" And so it remains today.

But I'd like to mention that the interjection Yo! in English might also be related to Io! in Latin, which also appears in Ancient Greek as ἰώ (iō, "oh!"). Reference: http://en.wiktionary.org/wiki/io

However, this connection (perhaps more related to the 1420 usage) might not have a continuous thread into modern U.S. usage.

For an example in Ancient Greek, see The Bacchae by Euripedes, where the devotees of Dionysus use Io! throughout the play, but more as an exclamation of joy and ecstasy.

Actually, the theory does sound very plausible. Here is a YouTube clip entitled Guaglione sung by the famossimo Neapolitan jazz/swing artist, Renato Carosone, you can hear the word being "sung" at 0.38.
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Mari-Lou AJun 8 '14 at 5:27

In Arabic, Ya is the vocative particle. That's the closest etymology I can think of. You can say: Ya Joseph, meaning O Joseph i.e. Yo Joseph! It's a way to call somebody out. This usage is at least 1800 years old and it stems from the Semitic tongue.

The phrase io Saturnalia was the characteristic shout or salutation of
the festival, originally commencing after the public banquet on the
single day of December 17.[16] The interjection io (Greek ἰώ, ǐō) is
pronounced either with two syllables (a short i and a long o) or as a
single syllable (with the i becoming the Latin consonantal j and
pronounced yō). It was a strongly emotive ritual exclamation or
invocation, used for instance in announcing triumph or celebrating
Bacchus, but also to punctuate a joke.[17]

Use of the term Yo! dates back to the Marine Corps work in Mexico with Pancho Villa in the early 1900s. The word yo means I in Spanish, and makes for a nice robust response at mail call or on pay day — any time when you want to be noticed. It is another piece of Marine history that is embraced by everyone.